New types of primary energy sources are needed to power electronic devices, electric vehicles and to smooth peak power demands on electric utilities. Promising devices for electrochemical energy conversion are based on the direct conversion of heat energy to electrical energy without use of moving mechanical parts.
Direct conversion of heat energy to electrical energy has both aesthetic and practical appeal. Several devices for thermoelectric direct conversion have been developed. The Seebeck effect, thermionic, and magnetohydrodynamic generators are familiar examples. None of these direct converters has been widely adopted because of practical problems such as parasitic heat loss or lack of a critical material with the physical properties necessary for high efficiency and good durability.
Among the less well known direct thermoelectric converters are the thermally regenerative electrochemical systems (TRES). These devices are closed electrochemical cells that produce electrical power. The reactants for these cells are regenerated within the device by thermal energy from a heat source which flows through the device to a heat sink. TRES have also been called electrochemical heat engines, by analogy with the well-known heat engines in which a working fluid is carried around a thermodynamic cycle. Many types of TRES were investigated in the 1950's and 1960's in the search for improved methods of converting the thermal output of nuclear reactors to electrical power.
All of the early TRES were plagued by practical problems such as inefficient heat exchange, electrode polarization, slow chemical regeneration kinetics, materials separation problems, and corrosion. Power densities of these early systems were usually limited to a few tens of milliwatts per square centimeter of electrode area, and thermoelectric efficiencies were below five percent.
A thermally powered sodium concentration cell based on a unique solid electrolyte was developed in 1968. This led to the development of the alkali metal thermoelectric converter (AMTEC) which is the first TRES with efficiency and power density comparable to conventional heat engines.
The alkali metal thermoelectric converter (AMTEC) is a device for the direct conversion of heat to electrical energy. The sodium ion conductor, beta-alumina, is used to form a high-temperature regenerative concentration cell for elemental sodium. An AMTEC can operate with an efficiency of 20 to 40 percent, a power density of 0.5 kilowatt per kilogram or more, while having no moving parts, low maintenance requirements, and high durability. Efficiency is independent of size. AMTEC devices should be usable with high-temperature combustion, nuclear, or solar heat sources. A wide range of applications from aerospace power to utility plants appears possible.
Improved power density from an AMTEC device was achieved by coating the beta-alumina with a 1-5 millimicron thick porous layer of a transition metal such as molybdenum. As disclosed in U.S. Pat. No. 4,175,164, the layer had good conductivity at the high temperature experienced in the device. Liquid sodium molybdate formed which facilitated sodium transport, as ions, through the porous electrode. However, only about 70 to 80 percent of theoretical efficiency was achieved and a two- to five-fold reduction in specific power output and efficiency were experienced after 10-1000 hours of operation. The voltage drop was related to electrode degradation. The flow resistance of sodium increased as sodium molybdate evaporated and less efficient gas diffusion of sodium through pores became the dominant transport process (3). The electrode was not capable of extended operation at high power levels. Many applications require operation of an AMTEC cell with porous electrodes at high specific power for periods of 10,000 hours or more.
Thus, the only remaining fundamental limitation of AMTEC devices is the provision of a long life (more than 10,000 hours) electrode with a negligible contribution to the internal impedance of the device.
Electrodes thinner than 1 millimicron would minimize the pressure difference across the electrode film and thus reduce sodium vapor transport resistance in the porous metallic film. However, very thin films present an increased sheet resistance that provides an excessive impedance to the AMTEC cell.
Very thin, porous molybdenum films having a thickness less than 1 millimicron and a current collector formed of fine, molybdenum grid lines connected by loops of molybdenum wire exhibit high power densities as disclosed in copending application entitled THIN METAL ELECTRODE FOR AMTEC filed concurrently herewith, the disclosure therein, being expressly incorporated herein by reference. AMTEC devices utilizing this electrode exhibit high power densities of over 0.3 watts/cm.sup.2 over extended periods of operation at high temperature. The thin electrodes allow efficient sodium vapor flow through the electrode, thus reducing the voltage loss. The thin film exhibits a high sheet resistance which is offset by the use of a current collector. The wired molybdenum grids provide an excessive contact resistance even when brazed to the thin film. Furthermore, the wired grids do not reliably adhere to the thin film.